Semiconductor structure with source/drain multi-layer structure and method for forming the same

ABSTRACT

A semiconductor structure and a method for forming the same are provided. The semiconductor structure includes a gate structure, a gate spacer and a source/drain structure. The gate structure is positioned over a fin structure. The gate spacer is positioned over the fin structure and on a sidewall surface of the gate structure. The source/drain structure is positioned in the fin structure and adjacent to the gate spacer. The source/drain structure includes a first source/drain epitaxial layer and a second source/drain epitaxial layer. The first source/drain epitaxial layer is in contact with the fin structure. The first source/drain epitaxial layer is connected to a portion of the second source/drain epitaxial layer below a top surface of the fin structure. The lattice constant of the first source/drain epitaxial layer is different from the lattice constant of the second source/drain epitaxial layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/586,272, filed Nov. 15, 2017, the entirety of which is incorporatedby reference herein.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapidgrowth. Technological advances in IC materials and design have producedgenerations of ICs where each generation has smaller and more complexcircuits than the previous generation. However, these advances haveincreased the complexity of processing and manufacturing ICs and, forthese advances to be realized, similar developments in IC processing andmanufacturing are needed. In the course of IC evolution, functionaldensity (i.e., the number of interconnected devices per chip area) hasgenerally increased while geometric size (i.e., the smallest componentthat can be created using a fabrication process) has decreased.

Despite groundbreaking advances in materials and fabrication, scalingplanar devices such as the metal-oxide-semiconductor field effecttransistor (MOSFET) device has proven challenging. To overcome thesechallenges, circuit designers look to novel structures to deliverimproved performance, which has resulted in the development ofthree-dimensional designs, such as fin-like field effect transistors(FinFETs). The FinFET is fabricated with a thin vertical “fin” (or finstructure) extending up from a substrate. The channel of the FinFET isformed in this vertical fin. A gate is provided over the fin to allowthe gate to control the channel from multiple sides. Advantages of theFinFET may include a reduction of the short channel effect, reducedleakage, and higher current flow.

However, since feature sizes continue to decrease, fabrication processescontinue to become more difficult to perform. Therefore, it is achallenge to form a reliable semiconductor structure including theFinFET.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It shouldbe noted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a three-dimensional view of an example simplified fin fieldeffect transistor (FinFET) in accordance with some embodiments;

FIG. 2, FIG. 3 and FIG. 4 are cross-sectional views along line A-A′ ofFIG. 1 showing various stages of a process for forming a semiconductorstructure, in accordance with some embodiments;

FIG. 5A is a cross-sectional view along line A-A′ of FIG. 1 showing astage of a process for forming a semiconductor structure after the stageshown in FIG. 4, in accordance with some embodiments;

FIG. 5B is a cross-sectional view along line B-B′ of FIG. 1 showing astage of a method of forming a semiconductor structure after the stageshown in FIG. 4, in accordance with some embodiments;

FIG. 6, FIG. 7, FIG. 8, FIG. 9, FIG. 10, FIG. 11 and FIG. 12A arecross-sectional views along line A-A′ of FIG. 1 showing various stagesof a process for forming a semiconductor structure after the stage shownin FIGS. 5A and 5B, in accordance with some embodiments;

FIG. 12B is a cross-sectional view along line B-B′ of FIG. 1 showing astage of a method of forming a semiconductor structure after the stageshown in FIG. 11, in accordance with some embodiments;

FIG. 13 is an enlarged view of FIG. 6, showing a blocking layer formedon a source/drain structure, in accordance with some embodiments;

FIG. 14 is a cross-sectional view of a semiconductor structure, inaccordance with some embodiments; and

FIG. 15 is a cross-sectional view of a semiconductor structure, inaccordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows includes embodiments in which the first and second features areformed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact.The present disclosure may repeat reference numerals and/or letters insome various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between somevarious embodiments and/or configurations discussed.

Furthermore, spatially relative terms, such as “beneath,” “below,”“lower,” “above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Some embodiments of the disclosure are described. Additional operationscan be provided before, during, and/or after the stages described inthese embodiments. Some of the stages that are described can be replacedor eliminated for different embodiments. Additional features can beadded to the semiconductor device structure. Some of the featuresdescribed below can be replaced or eliminated for different embodiments.Although some embodiments are discussed with operations performed in aparticular order, these operations may be performed in another logicalorder.

The fins may be patterned by any suitable method. For example, the finsmay be patterned using one or more photolithography processes, includingdouble-patterning or multi-patterning processes. Generally,double-patterning or multi-patterning processes combine photolithographyand self-aligned processes, allowing patterns to be created that have,for example, pitches smaller than what is otherwise obtainable using asingle, direct photolithography process. For example, in one embodiment,a sacrificial layer is formed over a substrate and patterned using aphotolithography process. Spacers are formed alongside the patternedsacrificial layer using a self-aligned process. The sacrificial layer isthen removed, and the remaining spacers may then be used to pattern thefins.

FIG. 1 illustrates a three-dimensional (3D) view of an example of asimplified fin field effect transistor (FinFET) 500 in accordance withsome embodiments. Other aspects not illustrated in or described withrespect to FIG. 1 may become apparent from the following figures anddescription. The FinFET 500 includes a fin structure 204 on a substrate200. The substrate 200 includes isolation regions 206, and the finstructure 204 protrudes above a top surface 208 of the isolation regions206. In addition, the fin structure 204 may be formed between theneighboring isolation regions 206. A gate structure 256 including a gatedielectric layer 252 and a gate electrode layer 254 is positioned overthe fin structure 204. The gate dielectric layer 252 is positioned alongsidewalls and over the top surface of the fin structure 204, and a gateelectrode layer 254 is positioned over the gate dielectric layer 252.Source/drain structures 220 are disposed in opposing regions of the finstructure 204 with respect to the gate dielectric layer 252 and the gateelectrode layer 254. FIG. 1 further illustrates a referencecross-section A-A′ and a reference cross-section B-B′ that are used forlater figures. The cross-section A-A′ may be in a plane along, e.g., achannel in the fin structure 204 between the opposing source/drainstructures 220. In addition, the cross-section B-B′ may be in a planealong, a width of the fin structure 204.

The source/drain structures 220 may be shared between varioustransistors, for example. In some examples, the source/drain structures220 may be connected or coupled to other FinFETs such that the FinFETsare implemented as one functional transistor. For example, ifneighboring (e.g., as opposed to opposing) source/drain regions areelectrically connected, such as through coalescing the regions byepitaxial growth, one functional transistor may be implemented. Otherconfigurations in other examples may implement other numbers offunctional transistors.

FIG. 2, FIG. 3 and FIG. 4 are cross-sectional views along line A-A′ ofFIG. 1 showing various stages of a process for forming a semiconductorstructure 600A, in accordance with some embodiments. FIG. 5A and FIG. 5Bare cross-sectional views along line A-A′ and line B-B′ of FIG. 1showing stages of a process for forming a semiconductor structure afterthe stage shown in FIG. 4. FIG. 6, FIG. 7, FIG. 8, FIG. 9, FIG. 10, FIG.11 and FIG. 12A are cross-sectional views along line A-A′ of FIG. 1showing various stages of a process for forming a semiconductorstructure after the stage shown in FIGS. 5A and 5B.

In some embodiments, a gate-replacement (gate-last) process is employedto fabricate the semiconductor structures 600A, such as a fin fieldeffect transistor (FinFET) (e.g. FinFETs 500A and 500B).

As shown in FIG. 2, the substrate 200 including the fin structure 204 isreceived. In some embodiments, the substrate 200 may be a semiconductorsubstrate, such as a bulk semiconductor, a semiconductor-on-insulator(SOI) substrate, or the like, which may be doped (e.g. with a P-type oran N-type dopant) or undoped. The substrate 200 may be a wafer, such asa silicon wafer. Generally, an SOI substrate includes a layer of asemiconductor material formed on an insulator layer. The insulator layermay be, for example, a buried oxide (BOX) layer, a silicon oxide layer,or the like. The insulator layer is provided on a substrate, typically asilicon or glass substrate. Other substrates, such as a multi-layered orgradient substrate may also be used. In some embodiments, thesemiconductor material of the substrate 200 may include silicon;germanium; a compound semiconductor including silicon carbide, galliumarsenic, gallium phosphide, indium phosphide, indium arsenide, and/orindium antimonide; an alloy semiconductor including SiGe, GaAsP, AlInAs,AlGaAs, GalnAs, GaInP, and/or GaInAsP; or combinations thereof.

In some embodiments, the fin structure 204 is formed by performing apatterning process on the substrate 200. The fin structure 204 may besurrounded by trenches (not shown) formed in the substrate 200 by thepatterning process. The isolation regions 206 (FIG. 1) may be formed ona bottom surface 210 of each of the trenches. A lower portion of the finstructure 204 is surrounded by the isolation structures, and an upperportion of the fin structure 204 protrudes from a top surface 208 ofeach of the isolation structures.

After the isolation regions are formed, dummy gate structures 215A and215B are formed over a top surface 205 of the fin structure 204, asshown in FIG. 2 in accordance with some embodiments. In addition, hardmask layers 214A and 214B are formed on the dummy gate structures 215Aand 215B, respectively. In some embodiments, the dummy gate structurescover respective channel regions of the resulting finFETs (e.g. theFinFETs 500A and 500B) on the fin structure 204. In some embodiments,the dummy gate structures 215A and 215B cover the top surface 205 andsidewalls of the fin structure 204, and extend over the isolation regionand the substrate 200 outside the fin structure 204.

In some embodiments, each of the dummy gate structures 215A and 215Bincludes a gate dielectric (not shown) and a gate electrode (not shown)formed over the gate dielectric. In some embodiments, the gatedielectric is silicon dioxide. In some embodiments, the silicon dioxideis a thermally grown oxide. In some embodiments, the gate dielectric isa high dielectric constant (high-k) dielectric material. A high-kdielectric material has a dielectric constant (k) higher than that ofsilicon dioxide. Examples of high-k dielectric materials include hafniumoxide, zirconium oxide, aluminum oxide, silicon oxynitride, hafniumdioxide-alumina alloy, hafnium silicon oxide, hafnium siliconoxynitride, hafnium tantalum oxide, hafnium titanium oxide, hafniumzirconium oxide, another suitable high-k material, or a combinationthereof. In some embodiments, the gate electrode includespolycrystalline-silicon (poly-Si), poly-crystalline silicon-germanium(poly-SiGe), metallic nitride, metallic silicide, metallic oxide, metal,and other suitable layers. In some embodiments, the gate electrode ismade of, for example, polysilicon.

In some embodiments, each of the hard mask layers 214A and 214B includesa single layer structure or a multi-layer structure. In someembodiments, the hard mask layers 214A and 214B are made of siliconoxide, silicon nitride, silicon oxynitride, silicon carbide, anothersuitable material, or a combination thereof.

In some embodiments, the formation of the dummy gate structures 215A and215B and the hard mask layers 214A and 214B includes depositionprocesses and a subsequent patterning process. The deposition processesare performed to deposit a gate dielectric material layer (not shown), agate electrode material layer (not shown) and a hard mask material (notshown) in sequence. The patterning process is then performed topartially remove the gate dielectric material layer, the gate electrodematerial layer and a hard mask material. Therefore, the dummy gatestructure 215A and the overlying hard mask layer 214A, and the dummygate structure 215B and the overlying hard mask layer 214B are formedover the fin structure 204. In some embodiments, the deposition processincludes a chemical vapor deposition (CVD) process, a physical vapordeposition (PVD) process, an atomic layer deposition (ALD) process, athermal oxidation process, or another applicable process. In someembodiments, the patterning process includes a photolithography processand a subsequent etching process. In some embodiments, the etchingprocess is a dry etching process.

After the dummy gate structures 215A and 215B and the hard mask layers214A and 214B are formed, a gate spacer layer 218 is formed over the finstructure 204, the dummy gate structures 215A and 215B and the hard masklayers 214A and 214B, as shown in FIG. 2 in accordance with someembodiments. In addition, the gate spacer layer 218 is conformallyformed over the dummy gate structures 215A and 215B. In someembodiments, the gate spacer layer 218 includes a single layer structureor a multi-layer structure. The gate spacer layer 218 may be made of lowdielectric constant (low-k) materials (e.g. k<5). In addition, the gatespacer layer 218 may be formed of oxide-free dielectric materials, suchas silicon nitride, silicon carbide, silicon carbonitride, anothersuitable material, or a combination thereof. The gate spacer layer 218may be deposited using a chemical vapor deposition (CVD) process, aphysical vapor deposition (PVD) process, a spin-on process, anotherapplicable process, or a combination thereof. The thickness of the gatespacer layer 218 may be in a range from about 1 nm to about 1 μm.

Afterwards, gate spacers 218A and 218B are formed on opposite sidewallsurfaces 315A and 315B of the dummy gate structures 215A and 215B andover the fin structure 204, as shown in FIG. 3 in accordance with someembodiments. Each of the gate spacers 218A and 218B may include a singlelayer structure or a multi-layer structure. In some embodiments, thegate spacers 218A and 218B are formed by an etching process. The etchingprocess is performed to remove the gate spacer layer 218 (FIG. 2) abovea top surface 217A of the hard mask layer 214A, a top surface 217B ofthe hard mask layer 214B and the top surface 205 of the fin structure204. In addition, the etching process is performed until the top surface205 of the fin structure 204 is exposed. In some embodiments, theetching process may include a dry etch process. The thickness of each ofthe gate spacers 218A and 218B may be in a range from about 1 nm toabout 1 μm.

After the gate spacers 218A and 218B are formed, an etching process 360is performed to remove portions of the fin structure 204 that are notcovered by the hard mask layers 214A and 214B, the dummy gate structures215A and 215B and the gate spacers 218A and 218B, as shown in FIG. 3 inaccordance with some embodiments. In some embodiments, the etchingprocess 360 is performed to form recesses 219A, 219B and 219C adjacentthe gate spacers 218A and 218B and in the fin structure 204. Inaddition, bottoms of the recesses 219A, 219B and 219C may be positionedbelow the top surface 208 of each of the isolation structures. Therecesses 219A, 219B and 219C are configured to provide positions of asource/drain structures formed in the subsequent processes. In someembodiments, the etching process 360 is a dry etching process. In someembodiments, etching gases used in the etching process 360 include HBr,NF₃, O₂ and other suitable etching gases.

Afterwards, first source/drain epitaxial layer 212A, 212B and 212C areepitaxial grown lining surfaces 211A, 211B and 211C of the fin structure204 in the recesses 219A, 219B and 219C, as shown in FIG. 4 inaccordance with some embodiments. In some embodiments, the firstsource/drain epitaxial layer 212A, 212B and 212C are conformally formedalong profiles of in the recesses 219A, 219B and 219C (i.e. the surfaces211A, 211B and 211C of the fin structure 204 in the recesses 219A, 219Band 219C) by an epitaxial growth process. In addition, the firstsource/drain epitaxial layer 212A, 212B and 212C are in contact with thefin structure 204. Furthermore, the first source/drain epitaxial layer212A, 212B and 212C may partially overlap the gate spacers 218A and218B.

In some embodiments, each of the first source/drain epitaxial layers212A, 212B and 212C may include a silicon epitaxial layer with a firstN-type dopant. For example, the first N-type dopant may include arsenic(As), carbon (C) or phosphorous (P). For example, the first source/drainepitaxial layers 212A, 212B and 212C may be formed of SiAs, SiCP, SiC,SiP or a combination thereof. The concentration of the first N-typedopant in the first source/drain epitaxial layers 212A, 212B and 212Cmay be in a range from 1E16 atoms/cm³ about to about 5E21 atoms/cm³. Insome embodiments, the epitaxial growth process includes an epitaxialprocess, such as a selective epitaxial growth (SEG) process, CVDdeposition techniques (e.g. vapor-phase epitaxy (VPE) and/or ultra-highvacuum CVD (UHV-CVD)), molecular beam epitaxy, or another suitableepitaxial process. The epitaxial growth process may be performed using aprecursor including AsH₃. The thickness of each of the firstsource/drain epitaxial layers 212A, 212B and 212C may be in a range fromabout 1 Å to about 300 nm.

After first source/drain epitaxial layers 212A, 212B and 212C areformed, the second source/drain epitaxial layers 213A, 213B and 213C areepitaxial grown over the first source/drain epitaxial layer 212A, 212Band 212C and filling the recesses 219A, 219B and 219C, as shown in FIG.5A and FIG. 5B in accordance with some embodiments. In some embodiments,the first source/drain epitaxial layer 212A and the overlying secondsource/drain epitaxial layer 213A collectively form a source/drainstructure 220A. Similarly, the first source/drain epitaxial layer 212Band the overlying second source/drain epitaxial layer 213B maycollectively form a source/drain structure 220B. The first source/drainepitaxial layer 212C and the overlying second source/drain epitaxiallayer 213C may collectively form a source/drain structure 220C. In someembodiments, the source/drain structures 220A, 220B and 220C are formedin the fin structure 204 and adjacent to the gate spacers 218A and 218B.The lattice constant of the first source/drain epitaxial layers 212A,212B and 212C are different from the lattice constant of the secondsource/drain epitaxial layers 213A, 213B and 213C. The lattice constantof the source/drain structures 220A, 220B and 220C are different fromthe lattice constant of the fin structure 204. In some embodiments, thesource/drain structures 220A, 220B and 220C may have a curved shape, adiamond shape, another applicable shape, or a combination thereof.

In some embodiments, the first source/drain epitaxial layers 212A, 212Band 212C are connected to portions of the second source/drain epitaxiallayers 213A, 213B and 213C below the top surface 205 of the finstructure 204. The portions of the second source/drain epitaxial layers213A, 213B and 213C below the top surface 205 of the fin structure 204are in contact with the first source/drain epitaxial layers 212A, 212Band 212C. In addition, the second source/drain epitaxial layers 213A,213B and 213C are separated from the fin structure 204 through the firstepitaxial layers 212A, 212B and 212C of the source/drain structures220A, 220B and 220C. Top surfaces 320A, 320B and 320C of the secondsource/drain epitaxial layers 213A, 213B and 213C may serve as topsurfaces 320A, 320B and 320C of the source/drain structures 220A, 220Band 220C. In addition, the top surfaces 320A, 320B and 320C of thesource/drain structures 220A, 220B and 220C may be positioned above orleveled with the top surface 205 of the fin structure 204. Furthermore,the source/drain structures 220A (or 220B, 220C) on neighboring finstructures 204 may be merged, as shown in FIG. 5B in accordance withsome embodiments.

In some embodiments, the second source/drain epitaxial layers 213A, 213Band 213C are formed of SiCP, SiC, SiP or a combination thereof. Inaddition, the composition of the second source/drain epitaxial layers213A, 213B and 213C may be different from the composition of the firstsource/drain epitaxial layers 212A, 212B and 212C. For example, each ofthe second source/drain epitaxial layers 213A, 213B and 213C may includea silicon epitaxial layer with a second N-type dopant. The second N-typedopant in the second source/drain epitaxial layers 213A, 213B and 213Cmay be different form the first N-type dopant in the first source/drainepitaxial layers 212A, 212B and 212C. In some embodiments, an atomicradius of the first N-type dopant is greater than an atomic radius ofthe second N-type dopant. Therefore, the lattice constant of each of thefirst source/drain epitaxial layers 212A, 212B and 212C is differentfrom (greater than) the lattice constant of each of the secondsource/drain epitaxial layers 213A, 213B and 213C of the source/drainstructures 220A, 220B and 220C. For example, when the first N-typedopant is arsenic (As), the second N-type dopant is phosphorous (P). Forexample, when the first source/drain epitaxial layers 212A, 212B and212C are formed of SiAs, the second source/drain epitaxial layers 213A,213B and 213C are formed of SiCP, SiC, SiP or a combination thereof.

In some embodiments, the second source/drain epitaxial layers 213A, 213Band 213C are formed by an epitaxial growth process. The epitaxial growthprocess may include an epitaxial process, such as a selective epitaxialgrowth (SEG) process, CVD deposition techniques (e.g. vapor-phaseepitaxy (VPE) and/or ultra-high vacuum CVD (UHV-CVD)), molecular beamepitaxy, or another suitable epitaxial process. The thickness of each ofthe second source/drain epitaxial layers 213A, 213B and 213C may be in arange from about 10 nm to about 1 μm. The thickness of each of thesecond source/drain epitaxial layers 213A, 213B and 213C may be greaterthan the thickness of each of the first source/drain epitaxial layers212A, 212B and 212C. In some embodiments, the thickness of each of thesecond source/drain epitaxial layers 213A, 213B and 213C may be oneorder to three orders of magnitude greater than the thickness of each ofthe first source/drain epitaxial layers 212A, 212B and 212C. Forexample, the ratio of the thickness of each of the second source/drainepitaxial layers 213A, 213B and 213C to the thickness of each of thefirst source/drain epitaxial layers 212A, 212B and 212C may be in arange from 10:1 to 1000:1.

In some embodiments, the first source/drain epitaxial layers 212A, 212Band 212C surround lower portions of the second source/drain epitaxiallayers 213A, 213B and 213C in the fin structure 204. In addition, thefirst N-type dopant in the first source/drain epitaxial layers 212A,212B and 212C may have an atomic radius larger than the atomic radius ofthe second N-type dopant in the second source/drain epitaxial layers213A, 213B and 213C. The first N-type dopant in the first source/drainepitaxial layers 212A, 212B and 212C may be hard to diffuse into the finstructure 204. In addition, the first source/drain epitaxial layers212A, 212B and 212C may prevent the second N-type dopant in the secondsource/drain epitaxial layers 213A, 213B and 213C from penetratingthrough the first source/drain epitaxial layers 212A, 212B and 212C andbeing diffused into the fin structure 204 (e.g. the channel region ofthe FinFET). Therefore, the drain induced barrier lowering (DIBL) effectcan be reduced. In addition, the electrical conductivity of the firstsource/drain epitaxial layers 212A, 212B and 212C (e.g., SiAs) may bebetter than the electrical conductivity of the second source/drainepitaxial layers 213A, 213B and 213C (e.g., SiAP). Therefore, onresistance (Ron) of the FinFET may be improved (reduced).

Afterwards, blocking layers 216A, 216B and 216C are formed on the topsurfaces 320A, 320B and 320C of the source/drain structures 220A, 220Band 220C, as shown in FIG. 6 in accordance with some embodiments. Forexample, portions of the source/drain structures 220A, 220B and 220C maybe transformed into the blocking layers 216A, 216B and 216C. Therefore,the top surfaces 320A, 320B and 320C of the source/drain structures220A, 220B and 220C is positioned below the blocking layers 216A, 216Band 216C. In addition, the blocking layers 216A, 216B and 216C may notbe formed on sidewall surfaces 268A and 268B of the gate spacers 218Aand 218B and the top surfaces 217A and 217B of the hard mask layers 214Aand 214B.

In some embodiments, each of the blocking layers 216A, 216B and 216Cincludes a polymer layer, such as a polysiloxane layer. The blockinglayers 216A, 216B and 216C may be formed by a gas phase depositionprocess 362, such as a chemical vapor deposition (CVD) process. Forexample, the gas phase deposition process 362 may be performed using aprecursor including methyl group (CH₃) in gas phase.

FIG. 13 is an enlarged view of portions 330 of FIG. 6. FIG. 13 shows theblocking layers 216A, 216B and 216C formed on the source/drainstructures 220A, 220B and 220C. In some embodiments, native oxide layers322 are formed on the source/drain structures 220A, 220B and 220C. Thenative oxide layers 322, for example, silicon dioxide (SiO₂), may beformed due to the exposure of the top surfaces 320A, 320B and 320C ofthe source/drain structures 220A, 220B and 220C. As shown in FIG. 13,the precursor (or the polymer form the precursor) and the native oxidelayers 322 may form Si—C bonds during the gas phase deposition process362. Therefore, the blocking layers 216A, 216B and 216C may be formed onthe top surfaces 320A, 320B and 320C of the source/drain structures220A, 220B and 220C after performing the gas phase deposition process362. When the gate spacers 218A and 218B and the hard mask layers 214Aand 214B are made of silicon nitride based materials, the blocking layermay not be formed on the sidewall surfaces 268A and 268B of the gatespacers 218A and 218B and on the top surfaces 217A and 217B of the hardmask layers 214A and 214B after performing the gas phase depositionprocess 362. In some embodiments, as shown in FIG. 13, each of theblocking layers 216A, 216B and 216C may include the native oxide layer322 and the methyl group.

Afterwards, dielectric spacers 270A and 270B are formed on sidewallsurfaces 268A and 268B of the spacers 218A and 218B by performing adeposition process 364, as shown in FIG. 7 in accordance with someembodiments. The dielectric spacers 270A and 270B are formed over thesource/drain structures 220A, 220B and 220C. In addition, the dielectricspacers 270A and 270B may be separated from the source/drain structures220A, 220B and 220C. During the deposition process 364, the blockinglayers 216A, 216B and 216C may serve as inhibitors to prevent thedielectric spacers 270A and 270B formed in contact with the top surfaces320A, 320B and 320C of the source/drain structures 220A, 220B and 220C.Therefore, the dielectric spacers 270A and 270B are selectively formedon the sidewall surfaces 268A and 268B of the spacers 218A and 218Brather than on the top surfaces 320A, 320B and 320C of the source/drainstructures 220A, 220B and 220C. In addition, the dielectric spacers 270Aand 270B may be positioned directly over the source/drain structures220A, 220B and 220C. In some other embodiments, the dielectric spacers270A and 270B may be formed covering the top surfaces 217A and 217B ofthe hard mask layers 214A and 214B.

The dielectric spacers 270A and 270B may be made of silicon oxide(SiO₂), silicon nitride (SiN), silicon oxynitride (SiON), silicon carbonoxynitride (SiCON), silicon carbonitride (SiCN), another dielectricmaterial or a combination thereof. The dielectric spacers 270A and 270Band the gate spacers 218A and 218B may be formed of different materials.For example, when the dielectric spacers 270A and 270B are formed ofsilicon nitride (SiN), the gate spacers 218A and 218B may be formed ofsilicon carbonitride (SiCN).

In some embodiments, the deposition process 364 includes atomic layerdeposition (ALD) process or another applicable process. The thickness ofeach of the dielectric spacers 270A and 270B may be in a range fromabout 1 Å to about 100 nm. The thickness of each of the gate spacers218A and 218B may be greater than the thickness of each of thedielectric spacers 270A and 270B. For example, the thickness of each ofthe gate spacers 218A and 218B may be one order to fourth orders ofmagnitude greater than the thickness of each of the dielectric spacers270A and 270B. For example, the ratio of the thickness of each of thegate spacers 218A and 218B to the thickness of each of the dielectricspacers 270A and 270B may be in a range from 10:1 to 10000:1.

In some other embodiments, the dielectric spacers 270A and 270B may beformed by using an implantation process to dope dopants in a portion ofthe spacers 218A and 218B from the (outer) sidewall surfaces 268A and268B of the spacers 218A and 218B. During the implantation process, theblocking layers 216A, 216B and 216C may serve as masks to prevent thedopants from being doped in the source/drain structures 220A, 220B and220C. The dopants used in the implantation process may include nitrogen,carbon or a combination thereof.

After the dielectric spacers 270A and 270B are formed, the blockinglayers 216A, 216B and 216C are removed from the top surfaces 320A, 320Band 320C of the source/drain structures 220A, 220B and 220C by a removalprocess 366, as shown in FIG. 8 in accordance with some embodiments. Thetop surfaces 320A, 320B and 320C of the source/drain structures 220A,220B and 220C are exposed after performing the removal process 366. Insome embodiments, the removal process 366 includes a baking process anda subsequent wet cleaning process. For example, the baking process mayhelp to break, for example, Si—C bond contained in the blocking layers216A, 216B and 216C. In some embodiments, the baking process maysublimate at least a portion of the blocking layers 216A, 216B and 216C,and the sublimated portion of the blocking layers 216A, 216B and 216Cmay be carried away by an applicable gas (e.g., H₂). The baking processmay be performed at a temperature in a range from about 200° C. to about400° C. For example, the wet cleaning process may include a sulfuricacid-hydrogen peroxide mixture (SPM) cleaning process to remove theremaining portion of the blocking layers 216A, 216B and 216C off thesource/drain structures 220A, 220B and 220C. The SPM solution may be a3:1 mixture of concentrated sulfuric acid (H₂SO₄) with hydrogen peroxide(H₂O₂).

Afterwards, dielectric spacers 272A and 272B are formed covering aportion of the (outer) sidewall surfaces 268A and 268B (FIG. 8) of thespacers 218A and 218B by a plasma etching process 368, as shown in FIG.9 in accordance with some embodiments. The plasma etching process 368may be performed with a tilt angle θ to remove a portion of thedielectric spacers 270A and 270B from top surfaces 274A and 274B of thedielectric spacer dielectric spacers 270A and 270B (FIG. 8). In someembodiments, the tilt angle θ may be larger than 15 degrees (e.g., 15degrees<θ<90 degrees) to avoid or reduce the damage to the source/drainstructures 220A, 220B and 220C during the plasma etching process 368. Insome embodiments, a lithography process may be performed to form a masklayer (not shown in the figures). The mask layer may cover and protectthe source/drain structures 220A, 220B and 220C but expose the portionof the dielectric spacers 270A and 270B intended to be removed by theplasma etching process 368. The dielectric spacers 272A and 272B may bepositioned directly over the second source/drain epitaxial layers 213A,213B and 213C of the source/drain structures 220A, 220B and 220C. Inaddition, top surfaces 276A and 276B of the dielectric spacers 272A and272B may be positioned between top surfaces (the position is leveledwith the interface between the dummy gate structure 215A (or 215B) andthe hard mask layer 214A (or 214B)) and bottom surfaces (the position isleveled with the top surface 205 of the fin structure 204) of the dummygate structures 215A and 215B. Furthermore, bottom surfaces 278A and278B of the dielectric spacers 272A and 272B may be positioned directlyabove the second source/drain epitaxial layers 213A, 213B and 213C. Insome other embodiments, the plasma etching process 368 is optional.

In some embodiments, the plasma etching process 368 may be performedusing a process gas including BF₂, BF₄, CF₄, O₂, SF₆, Cl₂, etc. Theplasma etching process 368 may be performed using a precursor includingBF₂, BF₄, CF₄, O₂, SF₆, Cl₂, etc. The plasma etching process 368 may beperformed at a temperature in a range from about 400° C. to about 600°C. In addition, the plasma etching process 368 may be performed with aradio-frequency (RF) power in a range from about 30 KW to about 1000 KW.

Afterwards, a dielectric layer 222 (such as an inter-layer dielectric(ILD) layer) is formed over the fin structure 204, the dummy gatestructures 215A and 215B, the gate spacers 218A and 218B, the dielectricspacers 272A and 272B, and the source/drain structures 220A, 220B and220C, as shown in FIG. 10 in accordance with some embodiments. Thedielectric layer 222 may fill gaps between the dummy gate structures215A and 215B. In addition, the top surfaces 276A and 276B of thedielectric spacers 272A and 272B are covered by the dielectric layer222.

In some embodiments, a deposition process is performed to form thedielectric layer 222. Afterwards, a planarization process is performedto level the top surfaces of the dielectric layer 222, the gate spacers218A and 218B, and the dummy gate structures 215A and 215B, as shown inFIG. 10.

In some embodiments, the dielectric layer 222 is made of a dielectricmaterial such as silicon oxide, phosphosilicate glass (PSG),borosilicate glass (BSG), boron-doped phosphosilicate glass (BPSG),undoped silicate glass (USG), carbon-doped silicate glass, siliconnitride or silicon oxynitride. In some embodiments, the dielectric layer222 is made of an extreme low-k (ELK) dielectric material with adielectric constant (k) less than about 2.5. With geometric sizeshrinking as technology nodes advance to 30 nm and beyond, ELKdielectric material is used to minimize device RC (time constant, R:resistance, C: capacitance) delay. In some embodiments, ELK dielectricmaterials include carbon doped silicon oxide, amorphous fluorinatedcarbon, parylene, bis-benzocyclobutenes (BCB), polytetrafluoroethylene(PTFE) (Teflon), or silicon oxycarbide polymers (SiOC). In someembodiments, ELK dielectric materials include a porous version of anexisting dielectric material, such as hydrogen silsesquioxane (HSQ),porous methyl silsesquioxane (MSQ), porous polyarylether (PAE), porousSiLK, or porous silicon oxide (SiO₂). In some embodiments, ELKdielectric material is deposited by a plasma enhanced chemical vapordeposition (PECVD) process or by a spin coating process.

In some embodiments, the deposition process of the dielectric layer 222includes a plasma enhanced chemical vapor deposition (CVD) process, alow pressure CVD process, an atomic layer deposition (ALD) process,flowable CVD (FCVD process), a spin-on coating process, or anotherapplicable process. In some embodiments, the planarization processincludes a chemical mechanical polishing (CMP) process, a grindingprocess, an etching process, another applicable process, or acombination thereof.

After the dielectric layer 222 is formed, metal gate structures 256A and256B are formed to replace the dummy gate structure 215A and 215B usinga removal process, a deposition processes and a subsequent planarizationprocess, and as shown in FIG. 10 in accordance with some embodiments. Insome embodiments, the metal gate structure 256A surrounded by the gatespacers 218A includes a gate dielectric layer 252A and a gate electrodelayer 254A over the gate dielectric layer 252A. Similarly, the metalgate structure 256B surrounded by the gate spacers 218B may include agate dielectric layer 252B and a gate electrode layer 254B over the gatedielectric layer 252B. In some embodiments, the gate spacers 218A arepositioned on opposite sidewall surfaces 255A of the metal gatestructure 256A, and the gate spacers 218B are positioned on oppositesidewall surfaces 255B of the metal gate structure 256B. In addition,the top surfaces 276A and 276B of the dielectric spacers 272A and 272Bmay be positioned between top surfaces 260A and 260B and bottom surfaces262A and 262B of the metal gate structures 256A and 256B.

In some embodiments, the gate dielectric layers 252A and 252B include asingle layer or multiple layers. In some embodiments, the gatedielectric layers 252A and 252B have a U-shape from a cross-sectionalview or a rectangular shape from a plane view. In some embodiments, thegate dielectric layers 252A and 252B are formed of silicon oxide,silicon nitride, or a high-k dielectric material (k>7.0) including ametal oxide or a silicate of Hf, Al, Zr, La, Mg, Ba, Ti, Pb, or acombination thereof. The formation methods of gate dielectric layers252A and 252B may include molecular beam deposition (MBD), atomic layerdeposition (ALD), plasma enhanced CVD (PECVD), and the like.

In some embodiments, the gate electrode layers 254A and 254B are made ofa metal-containing material such as TiN, TaN, TaC, Co, Ru, Al,combinations thereof, or multi-layers thereof, and are formed by adeposition process, such as electroplating, electroless plating, oranother suitable method.

In some embodiments, a work function layer (not shown) may be formed inthe metal gate structures 256A and 256B. The work function layer mayinclude N-work-function metal or P-work-function metal. The P-type workfunction layer may include TiN, TaN, Ru, Mo, Al, WN, ZrSi₂, MoSi₂,TaSi₂, NiSi₂, WN, another suitable P-type work function material, or acombination thereof. The N-type work function layer may include Ti, Ag,TaAl, TaAlC, TiAlN, TaC, TaCN, TaSiN, Mn, Zr, another suitable N-typework function material, or a combination thereof. In some embodiments,as shown in FIG. 10, the work function layer in the metal gatestructures 256A and 256B may include N-work-function metal.

Afterwards, a patterning process 370 is performed to form openings 232A,232B and 232C in the dielectric layer 222, as shown in FIG. 11 inaccordance with some embodiments. The openings 232A, 232B and 232C areformed passing through the dielectric layer 222 to expose thesource/drain structures 220A, 220B and 220C.

The patterning process 370 may include a photolithography process and asubsequent etching process. The photolithography process may beperformed to form a mask layer, which may be a photo-sensitive layersuch as photoresist, over the dielectric layer 226. The mask layer mayhave openings directly above to the positions of the source/drainstructures 220A, 220B and 220C. The photolithography process may includephotoresist coating (e.g. spin-on coating), soft baking, mask aligning,exposure, post-exposure baking, developing the photoresist, rinsing anddrying (e.g. hard baking). In some embodiments, the etching process is adry etching process. In addition, etching gases used in the etchingprocess include fluorine-containing (F-containing) gases. After theopenings 232A, 232B and 232C are formed, the mask layer may be removedby etching or any other suitable method.

In some embodiments, the dielectric spacers 272A and 272B are formed onthe outer sidewalls of the gate spacers 218A and 218B. The dielectricspacers 272A and 272B can help to increase the distance between thesubsequent contact plugs and metal gate structures. Therefore, thepatterning process 370 of the openings 232A, 232B and 232C may have awider process window. For example, bottom surfaces 234A, 234B and 234Cof the openings 232A, 232B and 232C may have wider widths (along thechannel length direction) while the widths of the openings 232A, 232Band 232C located close to a top surface of the dielectric layer 222 canbe kept. Therefore, the bottom surfaces 234A, 234B and 234C of theopenings 232A, 232B and 232C may be positioned lower the top surfaces320A, 320B and 320C of the source/drain structures 220A, 220B and 220C.In addition, the dielectric spacers 272A and 272B may be used to replacea contact etch stop layer (CESL).

Afterwards, source/drain silicide layers 240A, 240B and 240C are formedon the source/drain structures 220A, 220B and 220C in the openings 232A,232B and 232C by a silicidation process, as shown in FIGS. 12A and 12Bin accordance with some embodiments. As shown in FIG. 12A, thesource/drain silicide layers 240A and 240B may be separated from themetal gate structures 256A and 256B through the dielectric layer 222,the dielectric spacers 272A and 272B and the gate spacers 218A and 218Balong the longitudinal direction of the fin structure 204. In addition,as shown in FIG. 12A, the source/drain silicide layers 240A and 240B maybe separated from the gate spacers 218A and 218B through the dielectricspacers 272A and 272B along the longitudinal direction of the finstructure 204.

In some embodiments, the silicidation process includes a metal materialdeposition process and an annealing process performed in sequence. Insome embodiments, the deposition process of the silicidation processincludes a physical vapor deposition (PVD) process, an atomic layerdeposition (ALD) process, or another applicable process. In someembodiments, the annealing process of the silicidation process isperformed at a temperature in a range from about 300° C. to about 800°C. After the annealing process, the unreacted metal material is removed.

In some embodiments, the source/drain silicide layers 240A, 240B and240C are formed of one or more of cobalt silicide (e.g. CoSi, CoSi₂,Co₂Si, Co₂Si, Co₃Si; collectively “Co silicide”), titanium silicide(e.g. Ti₅Si₃, TiSi, TiSi₂, TiSi₃, Ti₆Si₄; collectively “Ti silicide”),nickel silicide (e.g. Ni₃Si, Ni₃₁Si₁₂, Ni₂Si, Ni₃Si₂, NiSi, NiSi₂;collectively “Ni silicide”), copper silicide (e.g. Cu₁₇Si₃, Cu₅₆Si₁₁,Cu₅Si, Cu₃₃Si₇, Cu₄Si, Cu₁₉Si₆, Cu₃Si, Cu₈₇Si₁₃; collectively “Cusilicide”), tungsten silicide (W₅Si₃, WSi₂; collectively “W silicide”),and molybdenum silicide (Mo₃Si, Mo₅Si₃, MoSi₂; collectively “Mosilicide”). The thickness of each of the source/drain silicide layers240A, 240B and 240C may be in a range from about 1 Å to about 500 nm.

In some embodiments, the bottom surfaces 234A, 234B and 234C of theopenings 232A, 232B and 232C (FIG. 11) may be positioned lower the topsurfaces 320A, 320B and 320C of the source/drain structures 220A, 220Band 220C. The source/drain silicide layers 240A, 240B and 240C may beformed in the positions leveled with or lower than the top surface 205of the fin structure 204. In addition, the dielectric spacers 272A and272B may serve as a block layer to prevent the silicide from extrudingfrom the subsequent source/drain silicide layer 240A, 240B and 240C tothe adjacent metal gate structures 256A and 256B.

Afterwards, contact barrier layers 242A, 242B and 242C are formedcovering sidewall surfaces and bottom surfaces of the openings 232A,232B and 232C (FIG. 11). The contact barrier layers 242A, 242B and 242Care formed covering the source/drain structures 220A, 220B and 220Cexposed by the openings 232A, 232B and 232C, as shown in FIGS. 12A and12B in accordance with some embodiments. In addition, contact plugs244A, 244B and 244C are formed filling the openings 232A, 232B and 232C(FIG. 11). The contact plugs 244A, 244B and 244C are formed passingthrough the dielectric layer 222 and positioned over the source/drainstructures 220A, 220B and 220C.

As shown in FIG. 12A, the contact barrier layers 242A, 242B and 242C maybe conformally formed over the source/drain silicide layers 240A, 240Band 240C and line the sidewall surfaces and the bottom surfaces of theopenings 232A, 232B and 232C (FIG. 11). In some embodiments, the bottomsurfaces of the contact barrier layers 242A, 242B and 242C arerespectively in direct contact with the source/drain silicide layers240A, 240B and 240C. In some embodiments, as shown in FIGS. 12A and 12B,the contact barrier layers 242A, 242B and 242C are respectively indirect contact with the first source/drain epitaxial layers 212A, 212Band 212C, and the contact barrier layers 242A, 242B and 242C arerespectively in direct contact with the second source/drain epitaxiallayers 213A, 213B and 213C.

As shown in FIG. 12A, sidewall surfaces 248A, 248B and 248C and bottomsurfaces 246A, 246B and 246C of the contact plugs 244A, 244B and 244Care covered by the contact barrier layers 242A, 242B and 242C,respectively. The dielectric spacers 272A and 272B surround portions ofthe sidewall surfaces 248A, 248B and 248C of the contact plugs 244A,244B and 244C. In addition, the contact plugs 244A, 244B and 244C areseparated from the metal gate structures 256A and 256B through thecontact barrier layers 242A, 242B and 242C, the dielectric layer 222,the dielectric spacers 272A and 272B and the gate spacers 218A and 218Balong the longitudinal direction of the fin 204. Furthermore, thecontact plugs 244A, 244B and 244C may be electrically connected to thesource/drain structures 220A, 220B and 220C. Therefore, the contactplugs 244A, 244B and 244C may serve as source/drain contact plugs.

In some embodiments, the contact barrier layers 242A, 242B and 242C andthe contact plugs 244A, 244B and 244C by deposition processes and asubsequent planarization process such as CMP. The contact barrier layers242A, 242B and 242C may include an electrically conductive material suchas titanium (Ti), titanium nitride (TiN), tantalum (Ta), tantalumnitride (TaN), nickel (Ni), nickel nitride (NiN), or the like, and maybe formed by a CVD process, such as plasma-enhanced CVD (PECVD).However, other alternative processes, such as sputtering or metalorganic chemical vapor deposition (MOCVD), physical vapor deposition(PVD), atomic layer deposition (ALD), may also be used. The contactplugs 244A, 244B and 244C may be made of a conductive material, such ascopper (Cu), aluminum (Al), tungsten (W), titanium (Ti), titaniumnitride (TiN), tantalum (Ta), tantalum nitride (TaN), or anotherapplicable material, and may be formed by any suitable depositionmethod, such as PVD, CVD, ALD, plating (e.g. electroplating). Thethickness of each of the contact barrier layers 242A, 242B and 242C maybe in a range from about 1 Å to about 20 Å. The height of each of thecontact plugs 244A, 244B and 244C may be in a range from about 20 Å toabout 500 nm.

After performing the aforementioned processes, a FinFET 500A and aFinFET 500B are formed. In some embodiments, the FinFET 500A and theFinFET 500B are N-type FinFETs. Moreover, a semiconductor structure 600Aincluding the FinFET 500A and the FinFET 500B is formed, as shown inFIG. 12A in accordance with some embodiments.

In some embodiments, the semiconductor structure 600A includes theN-type FinFET (e.g. the FinFET 500A and the FinFET 500B). Thesource/drain structure (e.g. the source/drain structures 220A, 220B and220C) of the N-type FinFET may be composed by the first source/drainepitaxial layer (e.g. the first source/drain epitaxial layers 212A, 212Band 212C) with the first N-type dopant and the second source/drainepitaxial layer (e.g. the second source/drain epitaxial layers 213A,213B and 213C) with the second N-type dopant. The first source/drainepitaxial layer may serve as a source/drain epitaxial liner layer incontact with the fin structure 204. The first source/drain epitaxiallayer may surround the lower portion of the second source/drainepitaxial layer in the fin structure 204. In addition, the atomic radiusof the first N-type dopant may be greater than the atomic radius thesecond N-type dopant. When the semiconductor structure 600A is processedin the thermal processes performed after the formation of thesource/drain structure, first N-type dopant (e.g. As) in the firstsource/drain epitaxial layer may suppress the second N-type dopant (e.g.P) in the second source/drain epitaxial layer diffusing into the finstructure 204. Because the atomic weight of the first N-type dopant isheavier than that of the second N-type dopant, the first N-type dopantis hard to diffuse into the channel region of the FinFET. Therefore, thedrain induced barrier lowering (DIBL) effect can be reduced. Inaddition, the electrical conductivity of the first source/drainepitaxial layers 212A, 212B and 212C (e.g., SiAs) may be better than theelectrical conductivity of the second source/drain epitaxial layers213A, 213B and 213C (e.g., SiAP). Therefore, on resistance (Ron) of theFinFET may be improved (reduced).

In some embodiments, the semiconductor structure 600A includes thedielectric spacer (e.g. the dielectric spacers 272A and 272B) on thelower portion of the outer sidewall surface (e.g. the sidewall surfaces268A and 268B) of the gate spacer (e.g. the gate spacers 218A and 218B).The dielectric spacer may be used to replace the CESL and help toincrease the process window of the contact hole (e.g. the openings 232A,232B and 232C in the dielectric layer 222). The distance between thecontact plug and the adjacent metal gate structure (e.g. the metal gatestructures 256A and 256B) may be further reduced. In addition, thesource/drain silicide layer (e.g. the source/drain silicide layers 240Aand 240B) may be formed in a lower position than the top surfaces of thesource/drain structure (e.g. the source/drain structures 220A, 220B and220C). Therefore, the dielectric spacer may serve as a block layer toprevent the silicide from extruding from the subsequent source/drainsilicide layer to the adjacent metal gate structure. The process yieldof the semiconductor structure 600A may be improved.

FIG. 14 is a cross-sectional view of a semiconductor structure 600B, inaccordance with some embodiments. The materials, configurations,structures and/or processes of the semiconductor structure 600B may besimilar to, or the same as, those of the semiconductor structure 600A,and the details thereof are not repeated herein. One of the differencesbetween the semiconductor device structure 600A and the semiconductordevice structure 600B is that dielectric spacers 372A and 372B areformed by another plasma etching process. The plasma etching process maybe performed to remove a portion of dielectric spacers 270A and 270Bfrom top surfaces 274A and 274B of the dielectric spacer dielectricspacers 270A and 270B (FIG. 8). In some embodiments, the conditions ofthe plasma etching process used for forming the semiconductor devicestructure 600B are different form the conditions of plasma etchingprocess 368. For example, the conditions of the plasma etching processmay include temperature, radio-frequency (RF) power, etc.

In some embodiments, the dielectric spacers 372A and 372B of thesemiconductor structure 600B are directly over the second source/drainepitaxial layers 213A, 213B and 213C of the source/drain structures220A, 220B and 220C. The dielectric spacers 372A and 372B may coverportions of the first source/drain epitaxial layer 212A, 212B and 212Cof the source/drain structures 220A, 220B and 220C. In addition, thedielectric spacers 372A and 372B may have rounded top portion 376A and376B due to the conditions of the plasma etching process.

In some embodiments, the dielectric spacers 372A and 372B of thesemiconductor structure 600B are positioned on the lower portions of theouter sidewall surfaces of the gate spacers (e.g. the gate spacers 218Aand 218B). The dielectric spacers 372A and 372B can help to increase thetotal volume of the gate spacers. In addition, the dielectric spacers372A and 372B may help to increase the process window of the contacthole (e.g. the openings 232A, 232B and 232C in the dielectric layer 222shown in FIG. 11). The distance between the contact plug and theadjacent metal gate structure (e.g. the metal gate structures 256A and256B) may be further reduced. Therefore, the dielectric spacers 372A and372B may be used to replace the CESL and help to facilitate thesource/drain silicide layer (e.g. the source/drain silicide layers 240Aand 240B) formed in a lower position than the top surfaces of thesource/drain structure (e.g. the source/drain structures 220A, 220B and220C). The dielectric spacers 372A and 372B may serve as a block layerto prevent the silicide from extruding from the subsequent source/drainsilicide layer to the adjacent metal gate structure. The distancebetween the contact plug and the adjacent metal gate structure (e.g. themetal gate structures 256A and 256B) may be further reduced. The processyield of the semiconductor structure 600B may be improved.

FIG. 15 is a cross-sectional view of a semiconductor structure 600C, inaccordance with some embodiments. The materials, configurations,structures and/or processes of the semiconductor structure 600C may besimilar to, or the same as, those of the semiconductor structure 600A,and the details thereof are not repeated herein. One of the differencesbetween the semiconductor device structure 600A and the semiconductordevice structure 600C is that a FinFET 500C and a FinFET 500D of thesemiconductor device structure 600C are P-type FinFETs. In addition, thework function layer in the metal gate structures 256A and 256B of theFinFETs 500C and 500D may include P-work-function metal. In someembodiments, source/drain structures 420A, 420B and 420C of thesemiconductor device structure 600C are formed of a silicon epitaxiallayer with a P-type dopant. For example, the P-type dopant in thesource/drain structures 420A, 420B and 420C may include boron (B).

In some embodiments, the dielectric spacer (e.g. the dielectric spacers272A and 272B) of the semiconductor structure 600C is selectively formedon the lower portion of the outer sidewall surface (e.g. the sidewallsurfaces 268A and 268B) of the gate spacer (e.g. the gate spacers 218Aand 218B). The dielectric spacer may be used to replace the CESL and mayfacilitate the source/drain silicide layer (e.g. the source/drainsilicide layers 240A and 240B) formed in a lower position than the topsurfaces of the source/drain structure (e.g. the source/drain structures420A, 420B and 420C). Therefore, the dielectric spacer may serve as ablock layer to prevent the silicide from extruding from the subsequentsource/drain silicide layer to the adjacent metal gate structure (e.g.the metal gate structures 256A and 256B). The process yield of thesemiconductor structure 600C may be improved.

As described previously, the N-type FinFET (e.g. the FinFET 500A and theFinFET 500B) of the semiconductor structure (e.g. the semiconductorstructure 600A and 600B) includes source/drain structure (e.g. thesource/drain structures 220A, 220B and 220C) composed by the firstsource/drain epitaxial layer (e.g. the first source/drain epitaxiallayers 212A, 212B and 212C) with the first N-type dopant and the secondsource/drain epitaxial layer (e.g. the second source/drain epitaxiallayers 213A, 213B and 213C) with the second N-type dopant. The firstsource/drain epitaxial layer may surround a portion of the secondsource/drain epitaxial layer in the fin structure 204. In addition, theatomic radius of the first N-type dopant (e.g. As) may be greater thanthe atomic radius the second N-type dopant (e.g. P). When thesemiconductor structure 600A is processed in the thermal processesperformed after the formation of the source/drain structure, the firstN-type dopant in the first source/drain epitaxial layer may suppress thesecond N-type dopant in the second source/drain epitaxial layerdiffusing into the fin structure 204 (e.g. the channel region of theN-type FinFET). Therefore, the drain induced barrier lowering (DIBL)effect can be reduced. In addition, the electrical conductivity of thefirst source/drain epitaxial layers (e.g., SiAs) may be better than theelectrical conductivity of the second source/drain epitaxial layers(e.g., SiAP). Therefore, on resistance (Ron) of the FinFET may beimproved (reduced).

As described previously, the semiconductor structure (e.g. thesemiconductor structures 600A, 600B and 600C) includes the dielectricspacer (e.g. the dielectric spacers 272A, 272B, 372A and 372B)selectively formed on a portion of the outer sidewall surface (e.g. thesidewall surfaces 268A and 268B) of the gate spacer (e.g. the gatespacers 218A and 218B). The dielectric spacer may be used to replace theCESL. Therefore, the distance between the contact plug (e.g. the contactplugs 244A, 244B and 244C) and the adjacent metal gate structure (e.g.the metal gate structures 256A and 256B) may be further reduced. Inaddition, the source/drain silicide layer (e.g. the source/drainsilicide layers 240A and 240B) may be formed in a lower position thanthe top surfaces of the source/drain structure (e.g. the source/drainstructures 220A, 220B, 220C, 420A, 420B and 420C). In addition, thedielectric spacer may serve as a block layer to prevent the silicidefrom extruding from the subsequent source/drain silicide layer to theadjacent metal gate structure. The process yield of the semiconductorstructure may be improved.

Embodiments of a semiconductor structure and a method for forming thesame are provided. The source/drain structure of the semiconductorstructure includes a first source/drain epitaxial layer and a secondsource/drain epitaxial layer over the first source/drain epitaxiallayer. The first source/drain epitaxial layer is in contact with the finstructure. The first source/drain epitaxial layer is connected to aportion of the second source/drain epitaxial layer below a top surfaceof the fin structure. The atomic radius of the first N-type dopant inthe first source/drain epitaxial layer may be greater than the atomicradius the second N-type dopant in the second source/drain epitaxiallayer. The first N-type dopant in the first source/drain epitaxial layermay suppress the second N-type dopant in the second source/drainepitaxial layer diffusing into the channel region of the FinFET. Thedrain induced barrier lowering (DIBL) effect can be reduced.

In some embodiments, a semiconductor structure is provided. Thesemiconductor structure includes a gate structure, a gate spacer and asource/drain structure. The gate structure is positioned over a finstructure. The gate spacer is positioned over the fin structure and on asidewall surface of the gate structure. The source/drain structure ispositioned in the fin structure and adjacent to the gate spacer. Thesource/drain structure includes a first source/drain epitaxial layer anda second source/drain epitaxial layer. The first source/drain epitaxiallayer is in contact with the fin structure. The second source/drainepitaxial layer is positioned over the first source/drain epitaxiallayer. The first source/drain epitaxial layer is connected to a portionof the second source/drain epitaxial layer below a top surface of thefin structure. The lattice constant of the first source/drain epitaxiallayer of the source/drain structure is different from the latticeconstant of the second source/drain epitaxial layer of the source/drainstructure.

In some embodiments, a semiconductor structure is provided. Thesemiconductor structure includes a gate structure, a gate spacer, asource/drain structure, a dielectric spacer and a contact plug. The gatestructure is positioned over a fin structure. The gate spacer ispositioned over the fin structure and on a sidewall surface of the gatestructure. The source/drain structure is positioned in the fin structureand adjacent to the gate spacer. The dielectric spacer is positionedover the source/drain structure and on a portion of a sidewall surfaceof the gate spacer. The contact plug is positioned over the source/drainstructure and separated from the gate spacer through the dielectricspacer. A top surface of the dielectric spacer is positioned between atop surface and a bottom surface of the gate structure.

In some embodiments, a method for forming a semiconductor structure isprovided. The method includes forming a gate structure over a finstructure. The method further includes forming a gate spacer over thefin structure and on a sidewall surface of the gate structure. Themethod further includes removing a portion of the fin structure notcovered by the gate structure and the gate spacer to form a recess inthe fin structure. The method further includes epitaxially growing afirst source/drain epitaxial layer lining a surface of the fin structurein the recess. The method further includes epitaxially growing a secondsource/drain epitaxial layer over the first source/drain epitaxial layerand filling the recess. The first source/drain epitaxial layer and thesecond source/drain epitaxial layer with different lattice constantsform a source/drain structure.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A semiconductor structure, comprising: a gatestructure over a fin structure; a gate spacer over the fin structure andon a sidewall surface of the gate structure; and a source/drainstructure in the fin structure and adjacent to the gate spacer, whereinthe source/drain structure comprises: a first source/drain epitaxiallayer in contact with the fin structure; and a second source/drainepitaxial layer over the first source/drain epitaxial layer, wherein thefirst source/drain epitaxial layer is connected to a portion of thesecond source/drain epitaxial layer below a top surface of the finstructure, wherein the lattice constant of the first source/drainepitaxial layer of the source/drain structure is different from thelattice constant of the second source/drain epitaxial layer of thesource/drain structure.
 2. The semiconductor structure as claimed inclaim 1, wherein the second source/drain epitaxial layer of thesource/drain structure is separated from the fin structure by the firstsource/drain epitaxial layer of the source/drain structure.
 3. Thesemiconductor structure as claimed in claim 1, wherein the firstsource/drain epitaxial layer of the source/drain structure comprises asilicon epitaxial layer with a first N-type dopant, and the secondsource/drain epitaxial layer of the source/drain structure comprises asilicon epitaxial layer with a second N-type dopant that is differentfrom the first N-type dopant.
 4. The semiconductor structure as claimedin claim 3, wherein an atomic radius of the first N-type dopant isgreater than an atomic radius of the second N-type dopant.
 5. Thesemiconductor structure as claimed in claim 4, wherein the first N-typedopant is arsenic (As), and the second N-type dopant is phosphorous (P).6. The semiconductor structure as claimed in claim 1, wherein thethickness of the second source/drain epitaxial layer of the source/drainstructure is one order to three orders of magnitude greater than thethickness of each of the first source/drain epitaxial layer of thesource/drain structure.
 7. The semiconductor structure as claimed inclaim 1, further comprising: a dielectric spacer over the source/drainstructure and on a portion of a sidewall surface of the gate spacer; anda contact plug over the source/drain structure, wherein the contact plugis separated from the gate spacer through the dielectric spacer.
 8. Thesemiconductor structure as claimed in claim 7, wherein a top surface ofthe dielectric spacer is positioned between a top surface and a bottomsurface of the gate structure.
 9. The semiconductor structure as claimedin claim 7, wherein the dielectric spacer is directly over the secondsource/drain epitaxial layer.
 10. A semiconductor structure, comprising:a gate structure over a fin structure; a gate spacer over the finstructure and on a sidewall surface of the gate structure; asource/drain structure in the fin structure and adjacent to the gatespacer; a dielectric spacer over the source/drain structure and on aportion of a sidewall surface of the gate spacer; and a contact plugover the source/drain structure and separated from the gate spacerthrough the dielectric spacer, wherein a top surface of the dielectricspacer is positioned between a top surface and a bottom surface of thegate structure.
 11. The semiconductor structure as claimed in claim 10,further comprising: a source/drain silicide layer on the source/drainstructure, wherein the source/drain silicide layer is separated from thegate spacer through the dielectric spacer along a longitudinal directionof the fin structure.
 12. The semiconductor structure as claimed inclaim 10, wherein the dielectric spacer is formed of a materialcomprising silicon nitride (SiN) and different from the gate spacer. 13.The semiconductor structure as claimed in claim 10, wherein thesource/drain structure comprises: a first source/drain epitaxial layerin contact with the fin structure; and a second source/drain epitaxiallayer over the first epitaxial layer, wherein the first source/drainepitaxial layer is connected to a portion of the second epitaxial layerbelow a top surface of the fin structure.
 14. The semiconductorstructure as claimed in claim 13, wherein the dielectric spacer isdirectly over the second source/drain epitaxial layer of thesource/drain structure.
 15. The semiconductor structure as claimed inclaim 13, wherein the first source/drain epitaxial layer of thesource/drain structure comprises a silicon epitaxial structure with afirst N-type dopant, and the second source/drain epitaxial layer of thesource/drain structure comprises a silicon epitaxial layer with a secondN-type dopant that is different from the first N-type dopant.
 16. Thesemiconductor structure as claimed in claim 13, wherein the firstsource/drain epitaxial layer of the source/drain structure comprisesSiAs, SiCP, SiC, SiP or a combination thereof.
 17. A method for forminga semiconductor structure, comprising: forming a gate structure over afin structure; forming a gate spacer over the fin structure and on asidewall surface of the gate structure; removing a portion of the finstructure not covered by the gate structure and the gate spacer to forma recess in the fin structure; epitaxially growing a source/drainstructure in the recess; forming a blocking layer on a top surface ofthe source/drain structure; selectively forming a dielectric spacer on asidewall surface of the gate spacer rather than on the top surface ofthe source/drain structure, wherein the gate spacer and the dielectricspacer are formed of different materials after forming the source/drainstructure; and removing the blocking layer from the top surface of thesource/drain structure.
 18. The method for forming a semiconductorstructure as claimed in claim 17, wherein epitaxially growing asource/drain structure in the recess comprises: epitaxially growing afirst source/drain epitaxial layer lining a surface of the fin structurein the recess; and epitaxially growing a second source/drain epitaxiallayer over the first source/drain epitaxial layer and filling therecess.
 19. The method for forming a semiconductor structure as claimedin claim 17, further comprising: performing a plasma etching process toremove a portion of dielectric spacer from a top surface of thedielectric spacer.
 20. The method for forming a semiconductor structureas claimed in claim 17, further comprising: forming an inter-layerdielectric (ILD) structure over the fin structure, the gate structure,the gate spacer, the source/drain structure and the dielectric spacer;forming a metal gate structure to replace the gate structure; andforming a contact plug over the source/drain structure.